In the description which follows, for the sake of easier and simpler illustration, reference will be made by way of an operating machine to a numerical-control machine tool for shaping slabs (for example of the type produced and marketed under the name “Contourbreton” by Breton S. p. A) using a tool mounted on a chuck displaceable in a controlled manner along a beam in turn displaceable relative to the work surface, so that the chuck and therefore the tool may be positioned opposite each point of the said surface.
The work surface consists of a table, preferably made of aluminium, on which the slabs to be machined are placed and temporarily fixed in position (for the time needed to performed the desired machining operations).
Clamping is usually performed by means of vacuum (suction cup) devices of variable shape and size, which are arranged in positions depending on the shape of the slab to be machined and in a number suitable for ensuring secure clamping of the slab also when subject to the stresses resulting from the action of the tool.
It should be noted in particular that the slabs to be machined must be clamped so as to leave between the bottom surface of the slab and the upper surface of the work surface sufficient space to allow penetration, into the slab, of tools (such as drilling bits or edge shaping tools, etc.) which must pass through the entire thickness of the slab.
Usually the machine also comprises a tool-holder store or crib and an automatic tool-changing mechanism.
Two operating methods are known and used to fix the slabs, i.e.:
(i) tables (or benches) are used, these having in their upper surface T-shaped grooves along which members for fixing simple single-acting suction cups are positioned and secured, said suction cups allowing clamping of the slab arranged on their upper ends;
(ii) the table is flat and continuous and has, positioned on its upper surface, double suction cup members, i.e. members which have two suction cups directed upwards and downwards respectively, so that the latter is intended to fix the member to the table, while the upper suction cup is used to fix in position the slab to be machined.
In any case, positioning of the suction cups is performed manually and necessarily involves the following operations:                determining the position in which each suction cup must be arranged (said position may be indicated by a light beam emitted, for example, by a laser projector);        manually arranging a suction cup in each of the given positions;        fixing the suction cup in the predefined position.        
If the suction cup is of the single-acting type, fixing is performed mechanically, after which the individual suction cups (i.e. their cavities) are connected to an active air suction circuit and therefore placed under a vacuum by means of flexible pipes.
Then the slab to the machined is placed on top of the suction cups and the air suction circuit is activated, creating a vacuum inside the cavities of the suction cups such as to retain and clamp the slab in the desired position.
If the suction cup is of the double-acting type, firstly each bottom suction cup, i.e. the suction cup resting on the work table or surface of the machine, is connected via a flexible pipe to a first active air suction circuit.
Then the bottom suction cups are fixed to the table, activating the first suction circuit so as to create a vacuum inside the cavities of the suction cups.
Then each upper suction cup is connected by means of a flexible pipe to a second air suction circuit independent of the first circuit.
The slab to be machined is placed on top of the upper suction cups of the two suction cups: if the machine is not equipped with an electronic feeler for detecting the exact position of the slab relative to the table, the slab is arranged so that at least one edge thereof comes into contact against suitable mechanical stops positioned on the edges of the table.
Finally the second air suction circuit is activated, namely the suction circuit which serves the cavities of the upper suction cups of each suction-cup pair, so that the slab is clamped in the desired position by the vacuum created inside the suction cups.
This procedure poses significant problems and drawbacks from an industrial point of view not only because it is long and laborious, but also because it requires action on the part of specialised labour.
Moreover, when the machining of each slab (for example forming part of a batch of slabs) has been completed, equally long operations are required not only in order to release the slab at the end of the machining operation, but also and in particular in order to rearrange the suction cups in the positions required for new slabs with different machining requirements.
Equally problematic is the presence of numerous small flexible pipes (two for each double-acting suction cup), for connecting the suction cups to the respective air suction source, not only as regards the space occupied on the machine table, but also with regard to the possibility of faults or malfunctions.